Methods for analyzing respirable particles in bulk materials

ABSTRACT

Provided is a method for detecting respirable participles in a bulk material comprising particles. The method comprises: analyzing morphology of the particles; analyzing chemical composition of the particles; creating a profile of the particles, wherein each particle in the profile is characterized by its shape, size and chemical composition; selecting particles from the profile which match the size and chemical composition of a respirable particle; and calculating a percentage of the respirable particles in the bulk material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalPatent Application 62/361,273 filed on Jul. 12, 2016, the entiredisclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention provides methods for accurate and rapid detection ofrespirable particles in bulk materials.

BACKGROUND

Many compounds used by the building construction industry are bulkmaterials, including, but not limited to, gypsum, calcined gypsum, mica,cement, calcium carbonate and sand. These dry compounds comprise apopulation of particles of different sizes.

Particles which are smaller than 20 microns are referred to respirableparticles because these particles can be dispersed into the air. Theseparticles can be then inhaled by workers, which should be avoided. Thus,there is a need to analyze a powder sample and provide an accurateestimate whether the sample comprises respirable particles and thepercentage of such particles in the sample.

The standard procedure for respirable silica requires the sample inanalysis to be treated with acid to eliminate most of the sample matrix.The remaining material is examined for crystalline silica. The weight ofcrystalline silica is determined using X-ray Diffraction or FTIR. Thisnumber is reported as Total Crystalline Silica. The second step requiresthe sample to be dispersed in alcohol and transferred to a silvermembrane and the mass of crystalline silica determined for the <10μfraction. A determination is also provided for the <5μ fraction.

The criteria in the Globally Harmonized System (GHS) have the finefraction of silica classified as specific target organ toxicity in thiscase, the lung. Generic cut-off values for products containing a finefraction of crystalline silica trigger a need for a method for thequantification of the fine fraction of crystalline silica in bulkmaterials.

The European Union has promoted an analysis for silica in bulk materialswhich culminated in the publication of a new standard for measuring theamounts of respirable particles in bulk materials. The standard has twoparts:

1. Determination of Size-Weighted Potential Respirable Fraction (SWeRF);and

2. Size-Weighted Potential Respirable Fraction of Crystalline Silica(SWeRF_(CS))

The method is to be used for comparing the potential health risks ofbulk materials. The method does not predict how a material will dispersein air, but quantifies the respirable fraction. The particles thatpresent a larger health risk are weighed more in the calculation. Theadvantage of the method is it provides an unambiguous characterizationof the bulk material. The term “potential” is used to indicate that thestandard does not analyze airborne particles.

The standard describes a method using sedimentation and a calculationmethod based on particle size distribution (PSD). The calculation canonly be used after the results are validated using the sedimentationdata. The calculation method requires that the particles have the samedensity or in the case of mixture with different materials that theyhave the same PSD. A plot is made to compare the sedimentation PSD withthe Stokes' Law and the convention described in CSN EN 481 (EuropeanStandards EN 481 “Workplace atmospheres—size fraction definitions formeasurement of airborne particles”). However, in this calculationmethod, the dynamic form factor is neglected where in the sedimentationmethod, the dynamic form factor is assumed to be equal in air andliquid. Thus, there remains a need for accurate and rapid detection ofrespirable particles in a bulk material.

SUMMARY

At least some of these needs are addressed by the present methods whichare based on analyzing particles for their morphology and chemicalcomposition and creating a profile for a sample in which each particleis characterized by its morphology and chemical composition.

This invention provides a method for detecting respirable participles ina bulk material comprising particles. The method comprises:

-   -   analyzing morphology of the particles;    -   analyzing chemical composition of the particles;    -   creating a profile of the particles, wherein each particle in        the profile is characterized by its shape, size and chemical        composition;    -   selecting particles from the profile which match the size and        chemical composition of a respirable particle; and    -   calculating a percentage of the respirable particles in the bulk        material.

In this method, the morphology and chemical composition of the particlescan be analyzed by a computer-controlled scanning electron microscopeinterfaced with an energy dispersive X-ray spectrometer (SEM□EDS). Themethod may comprise a step of resuspending the particles of the bulkmaterial, such as gypsum or calcium carbonate, in a medium (water ororganic solvent), and filtering the suspension through a filter with anominal pore size sufficiently small to retain the particles in therespirable size range. In the method, the particles can be retained on afilter. The morphology and chemical composition of the particles areanalyzed by an automated SEM□EDS. Various bulk materials can beanalyzed, including mixtures of inorganic compounds.

The present method can be used to analyze respirable particles, such asfor example silica (SiO₂), smaller than 20 microns. The present methodcan be also used to analyze respirable particles, such as for examplesilica (SiO₂), smaller than 10 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of methods for respirable particle analysis.

FIG. 2A is a plot for a bulk material comprising a small respirablefaction.

FIG. 2B are calculations in connection with the plot of FIG. 2A.

FIG. 3A is a plot for bulk material which does not comprise detectablerespirable silica.

FIG. 3B are calculations in connection with the plot of FIG. 3A.

FIG. 4A is a plot of a bulk material comprising a respirable fraction.

FIG. 4B are calculations in connection with the plot of FIG. 4A.

FIG. 5 is a scan of gypsum spiked with 0.5% of quartz.

FIG. 6 is a scan of gypsum spiked with 0.1% of quartz.

FIG. 7 is a plot with three calibration points.

FIG. 8 is a scan of selenite comprising quartz.

FIG. 9 is a first plot of filler Microwhite 100.

FIG. 10 is a first plot of filler Pulpro-20.

FIG. 11 is a pot of filler #3.

FIG. 12 is a second plot of filler Microwhite 100.

FIG. 13 is a second plot of filler Pulpro-20.

FIG. 14 is a plot of filler Snowhite 21.

FIG. 15 is a plot of filler S-200.

FIG. 16 is a plot of filler Marblehill.

FIG. 17 is a plot of filler G260 RM 71778.

FIG. 18 is an example of ESPRIT's Feature analysis workspace: a fractionof the filter area is imaged by SEM and converted to binary image, inwhich particles of interest (bright feature in the left frame) areidentified for further EDS analysis (right frame).

FIG. 19 is an example of automated particle analysis workspace: EDScomposition analyses of particles marked in FIG. 18 are in progress;note that results from particle sizing are tabulated below the fieldimage.

FIG. 20 is the particle analysis workspace: an example EDS X-rayspectrum acquired for one particle (most likely dolomite) identified inFIG. 18; note that results from EDS are tabulated below the spectrum.

FIG. 21 is a histogram of size (as AED) frequency of particles with Sirelative intensity >5% (cf. Table 4).

FIG. 22 reports Si contents in particles (with relative intensity >%5)as a function of size.

DETAILED DESCRIPTION

Provided is a method for examining the respirable fraction in bulkmaterials. The method comprises the following two steps. In step one, asample is analyzed for particle size distribution and density. This stepdetermines the total proportion of respirable particles in the sample.In step two, the amount of crystalline silica (also known as silicondioxide or quartz) in the sample is determined. Various matrices can beanalyzed by this method, including gypsum, cement, mica, calciumcarbonate, sand, etc. The method is illustrated in FIG. 1 in comparisonto other methods.

Step one can be performed by using a Quantachrome Pycnometer for adensity measurement, followed by an analysis with a particle sizeanalyzer (such as for example, Horiba LA-950V2) and the SWeRF equationwhich is validated for individual matrices. In step two, the mass ofsilica in the respirable fraction can be estimated by using a scanningelectron microscope which eliminates the need for X-Ray DiffractionInstrument. In further embodiments, a scanning electron microscopeinterfaced with an energy dispersive X-ray spectrometer can be used toanalyze particles for their morphology, including shape and size, andalso a chemical (elemental composition) of the particles. See FIG. 1 inwhich the present method is compared to other methods.

In the present method, a scanning electron microscope is used forcrystalline identification and morphology. This analysis can beconducted with computer software which captures data for each particleindividually, including the particle's shape, size and chemicalcomposition.

In one embodiment of the method, the SWeRF equations are evaluated usingthe CIC particle size analysis and RJ Lee Total Crystalline Silica. Atypical graphic output for plotting the PSD for the SWeRF calculation isshown in FIG. 2A. The plot indicates the DuPont Richmond FGD (trimodalPSD) has a small respirable fraction (1.6%). Using the SWeRF andSWeRF_(CS) equations as shown in the Table of FIG. 2B, the RJ Lee andSWeRF_(CS) estimate are in agreement and report 0.01% respirable silica.

As shown in FIG. 3A and the Table of FIG. 3B, the Dayton Power Light FGDillustrates a scenario where the PSD and SWeRF equations indicate norespirable fraction (monomodal PSD) and therefore no respirable silica.The RJ Lee data is indicating total crystalline silica of 0.2% and <10μrespirable silica of 0.2%.

As shown in FIG. 4A and the Table of FIG. 4B, the Montreal Recyclematerial illustrates a scenario where the PSD (bimodal) and SWeRFequations indicate a small respirable fraction in contrast to the RJ Leedata indicating a respirable silica fraction of 1.3%. This issignificantly higher than the 0.8% SWeRF estimate. This could requirethe sedimentation verification for SWeRF.

Table 1 is a summary of several different types of raw materials and howthe SWeRF estimates correlate with the RJ Lee respirable silica.

As shown in Table 1, in some examples (Dayton Power FGD, Rodemacher FlyAsh) there are two outputs. This occurs when the SDS gives a range forthe density. For example, the Rodemacher SDS provided a density range of2200-2800 kg/m³. The calculation for SWeRF was performed twice (2200 and2800 kg/m³).

TABLE 1 Sample name Haydite RM 81002 Date: Sep. 4, 2009 Sample Cryst.Silica cont. 33.9 % Sample identification Density = 623 kg/m3 CS Density= 2650 kg/m3 raw material (silica 50-60%) SWeRF = 46.2 % SWeRFcs = 9.2 %RJ Lee Total Crystalline Silica 33.9 RJ Lee Respirable Silica <10 μ 8.2RJ Lee Respirable Silica <5 μ 3.4 Sample Lansing Fly ash RM81141 Date:Aug. 20, 2010 Sample Cryst. Silica cont. 9.5 % Sample identificationDensity = 2000.0 kg/m3 CS Density = 2650.0 kg/m3 flyash density 2-3.2SWeRF = 25.4 % SWeRFcs = 2.2 % Sample Lansing Fly ash RM81141 Date: Aug.20, 2010 Sample Cryst. Silica cont. 9.5 % Sample identification Density= 3200.0 kg/m3 CS Density = 2650.0 kg/m3 flyash density 2-3.2 SWeRF =20.9 % SWeRFcs = 2.2 % RJ Lee Total Crystalline Silica 9.5 RJ LeeRespirable Silica <10 μ 2.6 RJ Lee Respirable Silica <5 μ 1.2 Sampleslate RM80062 Date: Sample Cryst. Silica cont. 16.4 % Sampleidentification Density = 1500 kg/m3 CS Density = 2650 kg/m3 slatedensity 1.47-1.53 SWeRF = 5.8 % SWeRFcs = 0.7 % RJ Lee Total CrystallineSilica 16.4 RJ Lee Respirable Silica <10 μ 0.2 RJ Lee Respirable Silica<5 μ N/A Sample mica RM79833 Date: Sample Cryst. Silica cont. 2.3 %Sample identification Density = 1800 kg/m3 CS Density = 2650 kg/m3 slatedensity 1.80 SWeRF = 7.8 % SWeRFcs = 0.1 % RJ Lee Total CrystallineSilica 2.3 RJ Lee Respirable Silica <10 μ 0.3 RJ Lee Respirable Silica<5 μ 0.2 Sample Termolita perlite Date: Aug. 18, 2010 Sample Cryst.Silica cont. 0.10 % (Microsil-200) RM81137 Sample identification Density= 2350 kg/m3 CS Density = 2650 kg/m3 perlite specific gravity SWeRF =2.4 % SWeRFcs = 0.002 % RJ Lee Total Crystalline Silica <0.1 RJ LeeRespirable Silica <10 μ <0.01 RJ Lee Respirable Silica <5 μ N/A SampleMicrosill 200S RM81249 Date: Jun. 14, 2011 Sample Cryst. Silica cont.0.3 % Sample identification Density = 2350 kg/m3 CS Density = 2650 kg/m3perlite specific gravity 2.35 SWeRF = 0.8 % SWeRFcs = 0.002 % RJ LeeTotal Crystalline Silica 0.3 RJ Lee Respirable Silica <10 μ 0.01 RJ LeeRespirable Silica <5 μ N/A DuPont Richmond RM80332 Date: Nov. 21, 2011Sample Cryst. Silica cont. 0.1 % Sample identification FGD GypsumDensity = 2320 kg/m3 CS Density = 2650 kg/m3 perlite specific gravity2.32 SWeRF = 1.6 % SWeRFcs = 0.001 % RJ Lee Total Crystalline Silica 0.1RJ Lee Respirable Silica <10 μ <0.01 RJ Lee Respirable Silica <5 μ <0.01DuPont Richmond RM80332 Date: Nov. 21, 2011 Sample Cryst. Silica cont.0.1 % Sample identification FGD Gypsum Density = 2960 kg/m3 CS Density =2650 kg/m3 perlite specific gravity 2.32 SWeRF = 1.182038043 % SWeRFcs =0.001 % RJ Lee Total Crystalline Silica 0.1 RJ Lee Respirable Silica <10μ <0.01 RJ Lee Respirable Silica <5 μ <0.01 Dayton Power Light RM80570Date: Jul. 2, 1997 Sample Cryst. Silica cont. 0.2 Sample identificationFGD Gypsum Density = 2300 kg/m3 CS Density = 2650 perlite specificgravity 2.3 SWeRF = 0.0 % SWeRFcs = 0.0 RJ Lee Total Crystalline Silica0.2 RJ Lee Respirable Silica <10 μ <0.05 Dayton Power Light RM80570Date: Jul. 2, 1997 Sample Cryst. Silica cont. 0.2 Sample identificationFGD Gypsum Density = 2500 kg/m3 CS Density = 2650 perlite specificgravity 2.3 SWeRF = 0.0 % SWeRFcs = 0.0 RJ Lee Total Crystalline Silica0.2 RJ Lee Respirable Silica <10 μ <0.05 Nepheline Syenite RM62074 Date:Jul. 2, 1997 Sample Cryst. Silica cont. 0.1 Sample identificationDensity = 2610 kg/m3 CS Density = 2650 perlite specific gravity 2.3SWeRF = 44.6 % SWeRFcs = 0.0 RJ Lee Total Crystalline Silica 0.1 RJ LeeRespirable Silica <10 μ <0.1 2015-134 Rodemacher fly ash C Date: Apr.24, 2015 Sample Cryst. Silica cont. 2 Sample identification fly ashDensity = 2200 kg/m3 CS Density = 2650 specific gravity 2.2 SWeRF = 10.8% SWeRFcs = 0.19 RJ Lee Total Crystalline Silica 2 RJ Lee RespirableSilica <10 μ 0.13 2015-134 Rodemacher fly ash C Date: Apr. 24, 2015Sample Cryst. Silica cont. 2 Sample identification fly ash Density =2800 kg/m3 CS Density = 2650 specific gravity 2.8 SWeRF = 9.3 % SWeRFcs= 0.19 RJ Lee Total Crystalline Silica 2 RJ Lee Respirable Silica <10 μ0.13 2015-137 Termolito perlite #1 Date: Apr. 27, 2015 Sample Cryst.Silica cont. 1.5 Sample identification perlite Density = 2350 kg/m3 CSDensity = 2650 specific gravity 2.35 SWeRF = 0.8 % SWeRFcs = 0.01 RJ LeeTotal Crystalline Silica 1.5 RJ Lee Respirable Silica <10 μ 0.192015-137 Termolito perlite #2 Date: Apr. 27, 2015 Sample Cryst. Silicacont. 1.9 Sample identification perlite Density = 2350 kg/m3 CS Density= 2650 specific gravity 2.35 SWeRF = 0.8 % SWeRFcs = 0.01 RJ Lee TotalCrystalline Silica 1.9 RJ Lee Respirable Silica <10 μ 0.3 RM 100125Gypsum Date: May 14, 2015 Sample Cryst. Silica cont. 3.6 Sampleidentification recycle Density = 2320 kg/m3 CS Density = 2650 wallboardspecific gravity 2.32 SWeRF = 0.80 % SWeRFcs = 0.03 RJ Lee TotalCrystalline Silica 3.6 RJ Lee Respirable Silica <10 μ 1.31

The XRD experiment on gypsum spiked with 0.5% and 0.1% quartz showsexcellent initial sensitivity, as shown in FIGS. 5 and 6. Even thoughthis only is three calibration points (FIG. 7), a scan of seleniteindicates the material is 0.056% quartz (FIG. 8).

Various carbonates which can be used as a filler were tested for SWeRFand SWeRFcs by the method described above. These values are provided inTable 2 below. See also FIGS. 9-17 for supporting plots.

TABLE 2 Estimated Respirable Fractions SWeRF = SWeRFcs = 2016-148 #180388-CP Filler 7.34 0.09 2016-148 #2 Marblewhite 310 8.30 0.0302016-148 #3 70283-BP-U 7.02 0.052 2016-159 #1 62303-Microwhite 100Sylacauga 9.63 0.12 2016-159 #2 66342-Pulpro-20 6.69 0.37 2016-159 #363305-Snowhite 21 8.73 0.33 2016-159 #4 70999 S-200 8.21 0.020 2016-159#5 60208 Microwhite 100 Marblehill 9.07 0.027 2016-182 #1 G260 RM 717787.75 0.024

Table 3 lists the particle size distribution for the carbonates.

TABLE 3 SMI Marble- Imerys Omya white CP-Filler BP-LU VOLUME DENSITY 310VOLUME DENSITY VOLUME DENSITY RUN (cc) (g/cc) RUN (cc) (g/cc) RUN (cc)(g/cc) 1 0.3353 2.9871 1 0.3569 2.8116 1 0.3603 2.7781 2 0.3332 3.0063 20.4005 2.5052 2 0.3588 2.7903 3 0.3366 2.9757 3 0.3612 2.7779 3 0.36032.7789 4 0.3354 2.9858 4 0.3621 2.7711 4 0.3639 2.7507 5 0.3392 2.9528 50.3642 2.7551 5 0.3667 2.7299 6 0.3384 2.9596 6 0.3646 2.7523 6 0.36892.7135 7 0.3397 2.9485 7 0.3659 2.7420 7 0.3707 2.7005 8 0.3391 2.9534 80.3257 3.0811 8 0.3727 0.6863 9 0.3413 2.9347 9 0.3703 2.7096 9 0.37452.6734 10 0.3405 2.9414 10 0.3700 2.7121 10 0.3722 2.6894 average 0.33792.9645 average 0.3641 2.7618 average 0.3669 2.7291 std. dev. 0.00260.0231 std. dev. 0.0181 0.1400 std. dev. 0.0058 0.0430 Imerys ImerysMicrowhite Microwhite 100- 100- Huber Sylacauga VOLUME DENSITYMarblehill VOLUME DENSITY G260 VOLUME DENSITY RUN (cc) (g/cc) RUN (cc)(g/cc) RUN (cc) (g/cc) 1 0.3666 2.7324 1 0.3694 2.7272 1 0.3683 2.7211 20.3670 2.7295 2 0.3711 2.7141 2 0.3689 2.7172 3 0.3700 2.7073 3 0.37412.6927 3 0.3691 2.7159 4 0.3702 2.7058 4 0.3768 2.6735 4 0.3699 2.7098 50.3705 2.7042 5 0.3789 2.6587 5 0.3688 2.7174 6 0.3731 2.6853 6 0.38042.6483 6 0.3714 2.6985 7 0.3731 2.6849 7 0.3822 2.6354 7 0.3682 2.7220 80.3742 2.6775 8 0.3815 2.6406 8 0.3709 2.7026 9 0.3729 2.6864 9 0.38202.6370 9 0.3675 2.7274 10 0.3744 2.6759 10 0.3789 2.6582 10 0.36832.7215 average 0.3712 2.6989 average 0.3775 2.6686 average 0.3691 2.7153std. dev. 0.0028 0.0204 std. dev. 0.0046 0.0327 std. dev. 0.0012 0.0091Omya JaJack Snowhite Omya S-200 VOLUME DENSITY 21 VOLUME DENSITY Pulpro20 VOLUME DENSITY RUN (cc) (g/cc) RUN (cc) (g/cc) RUN (cc) (g/cc) 10.3581 2.7958 1 0.3615 2.8165 1 0.3596 2.8497 2 0.3572 2.8031 2 0.36092.8212 2 0.3555 2.8826 3 0.3595 2.7851 3 0.3643 2.7949 3 0.3583 2.8598 40.3594 2.7854 4 0.3647 2.7913 4 0.3713 2.7598 5 0.3631 2.7573 5 0.36762.7693 5 0.3574 2.8677 6 0.3618 2.7672 6 0.3680 2.7663 6 0.3593 2.8518 70.3607 2.7758 7 0.3701 2.7512 7 0.3594 2.8513 8 0.3597 2.7831 8 0.37012.7508 8 0.3649 2.8083 9 0.3630 2.7583 9 0.3694 2.7562 9 0.3597 2.848710 0.3619 2.7668 10 0.3714 2.7415 10 0.3619 2.8320 average 0.3604 2.7778average 0.3668 2.7759 average 0.3607 2.8412 std. dev. 0.0020 0.0154 std.dev. 0.0037 0.0284 std. dev. 0.0045 0.0348

FIGS. 9-17 provide plots supporting the data in Table 3.

The data in Table 3 indicate that 5 carbonates out of all carbonateslisted in Table 2 meet the new silica limit. A person of skill will alsoreadily understand that the final contribution to the product of therespirable fraction depends on the formulation level.

Further embodiments provide a method in which particles are analyzedindividually to measure concentrations of respirable particles (such asfor example, silica, silicate minerals, asbestos, and any otherparticles that may be hazardous to a human if inhaled) in a bulkmaterial for safety assessment.

In this method, a sample of bulk material is dispersed and resuspendedin a suitable medium. The sample can be resuspended in water or in anorganic solvent, including, but not limited to, isopropanol or ethanol.The choice of a medium depends on the water solubility for a particularmaterial to be analyzed. For materials soluble in water, an organicsolvent is used.

A bulk material suspended in a medium can be subjected to filtrationthrough a membrane filter with pore sizes suitable for retainingparticles in the respirable size range. The respirable size range can beless than 20 μm in some applications, whereas in other applications, itcan be less than 10 μm. This method can be performed with 0.4-μmpore-sized polycarbonate filter to ensure all particles in therespirable size range are captured.

In one application of the present method, particles retained on themembrane filter by filtration are air-dried and coated with a thin layerof carbon before being subjected to analysis.

In the present method, an analysis of respirable particles is conductedby a scanning electron microscope (abbreviated as SEM) interfaced withan energy dispersive X-ray spectrometer (abbreviated as EDS). In thisanalysis, individual particles in a sample are analyzed for twodifferent properties:

-   -   a) morphological characteristics, including the particle's size        and shape; and    -   b) chemical (elemental) composition.

One suitable instrumental setup for the present method includes acomputer-controlled scanning electron microscope (SEM) interfaced withan energy dispersive X-ray spectrometer (EDS). This technique can beused to obtain accurate morphological (size, shape, etc.) and(elemental) compositional characterizations of thousands of individualparticles.

Morphology filters can be used to select a subset of detected particlesfor further EDS compositional analyses—only particles in the respirablesize range (e.g., <10 μm) can be selected if needed. This allows anadequate sampling and counting of deposited particles on the filter in atime-efficient manner.

Results of this analysis are shown in FIGS. 18□20. FIG. 18 depicts afraction of the filter area imaged by SEM and converted to a binaryimage, in which particles of interest (bright feature in the left frame)are identified for further EDS analysis (right frame).

FIG. 19 provides the EDS composition analysis of particles marked inFIG. 18. The results from particle sizing are tabulated below the fieldimage in FIG. 19.

FIG. 20 is an example of the EDS X-ray spectrum acquired for oneparticle (most likely dolomite) identified in FIG. 18. The results fromthe EDS analysis are tabulated below the spectrum in FIG. 20.

In the present method, a large number of particles can be accuratelyanalyzed for each particle's morphology and chemical composition. Thepresent method creates a multi-dimensional raw dataset (or a profile) ofthe particles for the sample. In this method, each particle ischaracterized by its morphological parameters (such as shape and size)and chemical composition.

In one application of the present method, a sample comprising 10% (byweight) respirable silica mixed in gypsum is analyzed for morphology andchemical composition of individual particles. A profile for this sampleis shown in Table 4 below. As can be seen from Table 4, the raw datasetmay include the following morphological characteristics of a particle:the area, aspect ratio, volume and diameter. In addition, the particle'schemical characteristics are represented by its elemental composition.This is particularly important for identifying different particle types(e.g., gypsum, quartz, calcium carbonate). In this example, eachparticle was analyzed for the presence of calcium (Ca), sulfur (S) andsilicon (Si).

TABLE 4 Selected output parameters from a computer-controlled SEM-EDSanalysis, for a sample made with known quantities of gypsum andrespirable silica mixed together. Area Volume (μm³)^(b) EquivalentRelative Elemental Particle Area Aspect Prolate Diameter X-rayIntensity^(c) ID (μm²) Ratio^(a) Sphere Spheroid (μm) Ca % S % Si %P4939 2.60 1.63 9.93 2.09 1.82 28.59 25.13 46.28 P4940 0.23 1.74 0.260.05 0.54 5.51 6.09 88.40 P4969 1.33 1.82 3.64 0.76 1.30 11.06 11.1877.76 P4973 3.20 1.56 13.54 2.72 2.02 24.57 21.71 53.72 P4975 2.01 1.406.71 1.70 1.60 44.10 43.48 12.42 P4979 0.17 1.52 0.16 0.03 0.46 37.2642.31 20.43 P4990 0.15 1.35 0.13 0.03 0.43 20.18 17.71 62.11 P4998 0.451.17 0.72 0.19 0.76 44.35 44.08 11.57 P5008 0.30 1.42 0.40 0.09 0.629.42 7.65 82.93 P5011 0.43 1.86 0.67 0.14 0.74 15.32 15.67 69.02 P50201.11 1.48 2.77 0.65 1.19 6.84 5.40 87.76 P5024 8.51 1.41 58.72 11.183.29 11.26 10.60 78.14 P5025 0.18 1.23 0.18 0.04 0.48 14.14 14.23 71.63P5031 0.37 1.85 0.53 0.10 0.68 18.05 18.92 63.03 P5036 0.36 1.28 0.500.12 0.67 32.83 30.99 36.18 P5043 0.30 1.84 0.40 0.08 0.62 13.98 15.1570.87 P5047 0.14 1.46 0.12 0.03 0.42 17.67 16.55 65.78 P5058 0.20 1.680.21 0.05 0.50 42.49 42.56 14.95 P5073 0.16 1.58 0.15 0.03 0.45 12.3110.66 77.03 P5076 0.49 1.75 0.82 0.15 0.79 9.93 10.71 79.36 P5090 10.241.95 77.40 15.98 3.61 47.22 41.82 10.95 ^(a)Aspect ratio = length/width,dimensionless; a measure of sphericity of a particle-1 for spheres, >1for irregular particles. ^(b)Particle volumes estimated for twodifferent shapes (later to be used for mass approximation with densityvalues for different composition. ^(c)Only relevant elements are shown;due to the use of polycarbonate filter membrane for sample preparation,oxygen (O) was not included in semi-quantification, and gypsum wasrepresented by the elemental presence of Ca and S. Silica wasrepresented by Si.

The large amount of information obtained for morphology and chemicalmakeup of particles affords a plethora of ways in which the raw data canbe analyzed. The following example (FIG. 21) presents the frequencydistribution of sizes (as area equivalent diameter, AED) of particleswith values of Si X-ray relative intensity >5%, for the same sample asin Table 4. Another way to present these data is to show the Si contentsin particles as a function of particle size (FIG. 22).

The present method may also include a step of further characterizing andgrouping particles according to a combination of particlecharacteristics listed (but not limited to) in Table 4. For example,normalized X-ray counts (net counts divided by particle size) can beused to differentiate crystalline and amorphous SiO₂.

Additional data analyses can be conducted with a specifically compliedcode program to selectively group particles according to theircharacteristics, including shape, size, chemical composition and anycombinations of parameters from the raw dataset.

The invention will be now described in more detail by the followingnon-limiting Examples.

Example 1

A small amount of weighed bulk material was well-mixed and suspended inknown volume (50-100 ml) of deionized water or isopropanol depending onthe water solubility of the material. An aliquot (generally <10 ml) waspressure filtered through 25-mm diameter, 0.4-μm pore-sizedpolycarbonate filter. Particles deposited on the membrane filter wereair-dried and coated with a thin layer of carbon before being subject toinstrument analysis.

An automated, computer-controlled particle analysis was conducted by ascanning electron microscope (SEM) interfaced with an energy dispersiveX-ray spectrometer (EDS). The automated particle analysis was used toprovide morphological (size, shape, etc.) and (elemental) compositionalcharacterizations of individual particles, results of which are shown inFIGS. 18-20. Morphology filters were used to select a subset of detectedparticles for further EDS compositional analyses. In this example, onlyparticles in the respirable size range (<10 μm) were selected.

Example 2

A sample comprising 10% (mass concentration) respirable silica mixed ingypsum (micronizing mill was used to homogenize the mixture) wasprepared. 8.4 mg of the mixture was suspended in 50 ml isopropanol; and3 ml of the suspension was filtered through 25-mm diameter, 0.4-μmpore-sized polycarbonate filter, resulting in 0.504 mg of materialretained on a deposition area of 3.14 cm².

The sample was analyzed as described in Example 1 and the particleanalysis data was tabulated in Table 4 and were also presented in FIGS.21 and 22.

In this analysis, particles containing ≥10%, Si (relative intensity) and≤10 μm were included in the calculation as respirable silica, anddensity value of 2.65 g/cm³ was used to estimate a particle mass,assuming a prolate spheroid particle shape. Knowing the fraction of thedeposition area analyzed by SEM-EDS, the mass concentration ofrespirable silica determined by the analysis was 9.8%, which isconsistent with that of the prepared sample mixture.

What is claimed is:
 1. A method for detecting respirable participles ina bulk material sample comprising particles, the method comprising:weighing the bulk material sample; resuspending the particles of thebulk material sample in a medium selected from water and/or organicsolvent, and filtering the suspension through a filter with a nominalpore size sufficiently small to retain the particles in the respirablesize range; analyzing morphology of the particles; analyzing chemicalcomposition of the particles; creating a profile of the particles,wherein each particle in the profile is characterized by its shape, sizeand chemical composition; counting the particles as a total number ofthe particles in the bulk material sample; selecting particles from theprofile which match the size smaller than 20 microns and chemicalcomposition of a respirable particle comprising silica (SiO₂); countingthe selected particles as respirable particles; and calculating apercentage of the respirable particles from the total number of theparticles in the bulk material sample.
 2. The method of claim 1, whereinthe morphology and chemical composition of the particles are analyzed bya scanning electron microscope interfaced with an energy dispersiveX-ray spectrometer.
 3. The method of claim 1, wherein the particles areretained on a filter, and wherein the morphology and chemicalcomposition of the particles are analyzed by a scanning electronmicroscope interfaced with an energy dispersive X-ray spectrometer. 4.The method of claim 1, wherein the bulk material is a mixture ofinorganic compounds.
 5. The method of claim 1, wherein the respirableparticles are smaller than 10 microns.
 6. The method of claim 1, whereinthe bulk material is selected from the group consisting of gypsum andcalcium carbonate.
 7. The method of claim 1, wherein the filter is apolycarbonate filter.